1. Field of the Invention
The present invention relates to the method of reducing heat radiation from the electrolytic alumina reduction cell. More particularly, it relates to a method of preventing the heat transfer from the bottom cathode of carbon blocks to the side wall made of carbon substrate by interposing the heat insulating substrate on the side surface of the cathode of the carbon blocks in the electrolytic alumina reduction cell.
2. Description of the Prior Arts
In order to prepare aluminum by the electrolysis of alumina, an electrolytic cell equipped with a carbon anode and a carbon cathode is used and 1 to 10% of alumina is melted in an electrolytic bath containing a main component of cryolite and current is fed to the bath at a current density of 0.5 to 1.0 A/cm.sup.2 whereby the alumina is electrolyzed.
The alumina is reduced as aluminum metal and the resulting aluminum metal is collected at the bottom of the electrolytic bath and it is intermittently discharged from the electrolytic cell.
The current of 50,000 to 200,000 Amp. is passed between the anode and the cathode of the electrolytic alumina reduction cell. In the electrolytic bath, Joule's heat caused by the current is generated whereby the temperature of the bath is maintained at higher than the temperature for melting the bath preferably about 950.degree. to 980.degree. C.
When the alumina in the electrolytic bath is reduced to form aluminum and the concentration of alumina in the bath is reduced, suitable amount of alumina is usually fed from both sides or from the horizontal central axis in longitudinal direction of the electrolytic cell above the electrolytic bath.
When alumina is fed into the electrolytic bath, the temperature of the bath at the part is lowered by the alumina fed to the part. Sometimes, the bath is solidified to cause a solid bath in the electrolytic bath.
When the alumina is fed into the bath from both sides in the longitudinal direction of the electrolytic cell, the solid bath is formed to cover the whole inner wall of both sides in the longitudinal direction.
The solid bath protects the corrosion of the inner wall caused by the electrolytic bath at high temperature and it prevents the heat radiation from the inner part of the electrolytic cell.
However, in said case, the alumina is not fed to both sides in the short direction of the electrolytic cell, whereby the solid bath is scarcely formed on the inner wall at the both sides in the short direction. Accordingly, the inner wall is corroded by the electrolytic bath and a large quantity of heat is radiated from the inner part of the electrolytic cell through the parts out of the cell.
When the alumina is fed above the central axis in the longitudinal direction of the electrolytic cell, the solid bath is not substantially formed on the inner wall at the both sides in the longitudinal direction as well as the inner wall at the both sides in the short direction. Accordingly, the corrosion of the inner side wall caused by the electrolytic bath is quite high and the heat radiation from the inner part of the electrolytic cell is remarkably large.
The inventors have studied on the method of forming the solid bath on whole of the inner side wall of the electrolytic cell in order to protect the inner side wall from the corrosion caused by the electrolytic bath and to prevent a large quantity of heat radiation from the cell. As the result, the present invention has been attained by finding the fact that suitable solid bath can be formed on the inner side wall of the electrolytic cell by interposing a heat insulating substrate at the specific position of the cathode at the bottom of the electrolytic cell.